When constructing wells such as an oil or gas well a borehole is drilled opening the differently pressured formations to some fluid communications. During the drilling phase a drilling fluid or ‘mud’ is maintained at a density high enough into the well to prevent communication between the different zones. Once the required depth is reached a steel casing or liner is lowered into the well and cemented into position. Liquid cement is pumped down through the casing and displaced in the annulus between the casing and formation, where it is left to set. This setting process takes a considerable time, for example several hours, typically 24 hours or more. While the cement is setting no further work can be conducted on the borehole to avoid moving the casing while it is being set in position.
Many other oilfield cementing operations such as setting cement plugs, kick-off-plugs or cement squeezes are performed every day with similar cement slurry systems to the one used for primary casing or liner cementing. Traditionally cement systems based on Portland cements are being used. Different qualities of Portland cements have been developed over time to answer the particular needs of oilfield cementing and in particular the temperature and pressure effects on the set of the cement and its performance once set in place. Most typical cement grades are referred as A, G or H types according to the American Petroleum Institute (API).
The drilling operation is a continuous and costly 24 hour a day operation. This is particularly the case on offshore platforms or deepwater operations where very large daily costs are being spent. While drilling runs continuously, the cementing operations require time to set and this is called “wait on cement” (WOC) time for the cement to set hard after displacement into the well. Usually it is good practice to try and minimize this WOC time as much as possible. There is therefore a duality between the need and desire to retard the setting of the cement sufficiently to allow for its safe placement as a liquid into the well so it can adopt the required position, and the need and desire to wait as little time on the cement to set hard for economical reasons.
Portland cement systems have proved efficient for most cementing oilfield operations. Portland cement has nevertheless shown some limitations in some particular well configurations where the resultant very long WOC times made it difficult for the cement to be placed and still become hard (set) along the entire column in an acceptable amount of time. Sometimes the temperature difference between the top of the cement column and the temperature at the shoe is such that the cement never sets at the top of the column. Remedial cement jobs then need to be performed at the expense of further time and money.
Portland cement has a well known limited performance when it comes to set-cement mechanical properties. Not all oil or gas wells show similar configurations and stresses from the wellbore onto the steel casings and cement sheath attached to it are varied. Sometimes the cement used is not hard enough. This performance relates to the ‘compressive-strength’ of cement. In other well configurations the cement is not ductile enough to both absorb the stress-changes during the life of a well and deform without failure i.e. developing cracks. In these latter conditions, the industry has tried to improve the ‘ductility’ of the set-cement by lowering its Young's Modulus or modifying its Poisson Ratio.
In some other configurations, it was shown that the failure mode of the set-cement sheath during the life of the well could be attributed to the tensile stresses on the cement sheath. Portland cement systems exhibit sufficiently high compressive strength in most cases to be a suitable material for use in oilfield wells, but it has not been found to be satisfactory in terms of tensile strength or Young's Modulus, Poisson ratio etc.
There is a need to provide cement systems which exhibit better set-cement mechanical properties and do not fail during the life of the well. It would also be advantageous to use systems that require shorter WOC times. This is particularly the case for applications such as cement plugs and kick-off plugs where shorter WOC is paramount as the operations may have to be repeated. It is a known fact that the success ratio of setting e.g. cement kick-off plugs is less than 1 out of 2 performed. Many times, failure to kick-off properly has been attributed to a lack of compressive-strength of the cement placed in the well. Having a cement system that would improve this success ratio by developing higher earlier compressive strength has considerable advantages both in terms of performance and economy.
Failure to place cement properly to provide zonal isolation is sometimes related to the difficulty to retard satisfactorily a cement slurry and achieve at the same time sufficient compressive-strength. Conventional Portland cement systems may be especially sensitive to temperature variations under certain ranges of temperatures like 90-130° C. When a cement system is retarded for temperature in the 120-130° C. range, it has difficulties to set at e.g. 90° C. This over retardation effect is detrimental to zonal isolation and sometimes needs repairs after the primary cementing operation. It is therefore a need for a cement system which has reduced sensitivity to temperature variations compared to traditional Portland based cements.
In the systems of the prior art there are also difficulties encountered with preparing the cement slurries at the location. Typically this has involved mixing the correct ratio of dry cement and a pre-mixed water including desired additional chemicals like retarders, dispersants etc. Achieving the correct density throughout the cement job is a challenge considering the irregular pneumatic flow of the cement blend or other operational considerations. It is therefore a need for a cement system which exhibits less sensitivity to density variations than Portland cement systems.
In systems of the prior art, large fluid loss under borehole condition is commonly experienced, even with a fluid loss additive added to reduce fluid loss, cementing job might fail due to wrong estimation of down hole environment. So there is a need for a cement system with good inherent fluid loss control mechanism, e.g. closed particle packing to form dense filter cake, thereby stopping fluid loss.